398
FXUS63 KTOP 051125
AFDTOP
AREA FORECAST DISCUSSION
National Weather Service Topeka KS
525 AM CST Mon Dec 5 2016
...Update to aviation forecast discussion...
.SHORT TERM...(Today and tonight)
Issued at 148 AM CST Mon Dec 5 2016
Zonal flow aloft and a shortwave over the northern Rockies has
allowed return flow to develop across the region. The pressure
gradient is increasing from west to east this morning, but is
somewhat weak over eastern KS. This along with clear skies
supported fog development mainly in the low lying areas. Although
the models are indicating increasing winds just above the surface,
which should keep the boundary layer mixed. This is already taking
place in central KS therefore do not expect the temperatures to
decrease much below the current conditions. The surface wind
appeared to decouple last evening for a brief period, which caused
patchy fog to form. This probably allowed the wind above the
surface to increase enough to disrupt the decoupling and fog
sustainability.
Some of the high res models have been struggling with this
surface mixing and therefore are overestimating the low level
saturation. This has caused the models to forecast fog and stratus
later this morning mainly over east central KS. Current
observations do not show any stratus in southeast KS and northeast
OK, and the dew points moving into the area are not much different
than the current. There is some evidence of some moisture
advection in the lowest layers above the surface, which could
support development. If stratus were to form it could affect
temperatures depending on how long it lingered today. Confidence
in this stratus building into the area this morning is not high so
will continue to monitor the latest guidance and observations.
Within the return flow today southerly winds will be 10 to 20 mph
with gusts up to 30 mph. Highs should manage the lower to mid 50s.
Winds will decrease in the evening as a cold front pushes through
the area, but will quickly increase out of the northwest tonight
along and behind the front. Cooler temperatures then filter in
from the north and lows drop into the mid to upper 20s.
.LONG TERM...(Tuesday through Sunday)
Issued at 148 AM CST Mon Dec 5 2016
By early Tuesday, cold front continues to make its way southward
into the Central Plains. Highs are forecast in the 30s to near 40
in the southeast, but doubt these will last long before falling as
the colder air moves in. Reinforcing shot of cold air is swept
southward behind the upper low over the Great Lakes on Tuesday
night, and highs don`t make it out of the 20s for Wednesday as a
result. Colder air makes for higher snow/liquid ratios for snow
chances on Wednesday, running generally in the 15-18 to 1 range.
Models still generating different snow amounts for this period.
Cross sections depict area of frontogenesis, with a window of weak
elevated instability aloft around 18z, and battle seems to be with
how much of the precip is eaten up by drier air near the surface
before it can start to accumulate. While there are still some
outliers, most ensemble members are averaging about 1-3 inches of
snow across the area, with some outliers on both the higher and
lower ends. Would not be surprised if some of this snow fell
quickly given lift/instability potential and high ratios, so still
need to be mindful of possible impacts if a few inches fall in a
short period of time during the daytime hours - with best
potential in the morning to early afternoon hours. With
differences still in the forecast, stay tuned for updates.
Much colder airmass continues to move southward through Wednesday
night and Thursday, with lows in the single digits and highs
Thursday in the teens to near 20. Wind chills in the overnight
hours fall into the 0 to 5 below range, which doesn`t take much
wind given ambient air temps. Thursday and Friday mornings will be
a very cold start, plan accordingly.
Although highs on Friday afternoon are forecast in the 20s, a
quick turn around to southerly return flow under westerly flow
aloft bumps overnight lows back up into the 20s. This brings high
temperatures, and southerly winds, back up on Saturday as well
to around 40F. End of the forecast guidance for Sunday noses some
cooler air back into northern Kansas, with much colder air
remaining poised across Southern Canada and the Northern Plains.
&&
.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 521 AM CST Mon Dec 5 2016
Fog development has been limited this morning as the surface
remains mixed. Also, the stratus has not formed as previously
thought. There is a very slight chance some clouds could form as
the surface moisture mixes out later this morning. Later tonight a
front will move through the area causing a wind shift. Borderline
MVFR ceilings will be possible tonight through the end of the
period.
&&
.TOP WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SHORT TERM...Sanders
LONG TERM...67
AVIATION...Sanders